One conventional method for testing for the presence of an analyte in a test solution comprises capturing the analyte on a dipstick and detecting for the presence of the analyte on the dipstick. The dipstick has a contact end for contacting the test solution and a capture zone remote from the contact end to which an anti-analyte antibody (the capture antibody) is immobilized.
To test for the presence of analyte, the contact end of the dipstick is contacted with the test solution. If analyte is present in the test solution it travels to the capture zone of the dipstick by capillary action where it is captured by the capture antibody. The presence of analyte at the capture zone of the dipstick is detected by a further anti-analyte antibody (the detection antibody) labelled with, for example, colloidal gold.
These dipstick tests have several advantages. They are easy and cheap to perform, no specialist instruments are required, and the results are obtained rapidly and can be read visually. These tests are, therefore, particularly suited for use in a physician's office, at home, in remote areas, and in developing countries where specialist equipment may not be available. They can be used, for example, to test whether a patient is infected with a disease causing micro-organism such as Chlamydia trachomatis. 
However, the sensitivity of analyte detection using such tests is relatively low. Consequently, if the analyte is only present in small amounts in the test solution it can remain undetected. This is a particular disadvantage if the test is being used to diagnose whether or not a patient has a particular disease as the patient may not be diagnosed as having that disease. This is particularly the case where the patient is asymptomatic, but also where symptoms are present but it is necessary to confirm that these are caused by a particular disease, or particular disease strain.
WO 00/25135 discloses a two-step dipstick detection system for detecting antigen. The detection system uses an anti-antigen biotinylated antibody and an anti-biotin antibody labelled with colloidal gold (anti-biotin gold conjugate). In the first step, the biotinylated antibody is mixed with the test solution and the tip of the dipstick membrane is then immersed in the mixture so that the aqueous mixture wicks up the membrane by capillary action. Antigen in the mixture bound to the biotinylated antibody is captured by an immobilised anti-antigen antibody at a capture zone of the dipstick to form a complex comprising the immobilised anti-antigen antibody, the antigen and the biotinylated antibody. In the second step, the dipstick is immersed in a separate suspension of anti-biotin gold conjugate. The anti-biotin gold conjugate wicks up the membrane by capillary action and binds to biotinylated antibody captured with the antigen at the capture zone. Antigen is then detected by the presence of gold label at the capture zone.
Whilst the two-step system may provide increased sensitivity, it is desired to further improve the sensitivity of analyte detection.